Vacuum fluorescent display tubes for displaying numerals or other characters have been widely used, such as in computers, measuring apparatus and calculators, to provide a visual display of information. The tubes are conventionally formed such that filaments, a grid electrode, anode segments, and a pair of filament supporters are placed within an evacuated envelope which is formed by a spacer interposed between a pair of transparent substrates made of glass.
A typical fluorescent display tube is described in connection with FIG. 3.
Distributing wires (not shown) for supplying voltage, anode segments 8, a grid electrode 10 and filaments 12 are layered on a lower substrate 4. A pair of metal filament supporters 14 are placed thereon at opposite ends of substrate 4 at a predetermined distance from each other to support each end of the filaments. Supporter 14 is welded and connected to a base 15 fixed on the lower substrate 4 by a print 16. Anode segments 8 are formed thereon in a predetermined pattern. Each anode segment is covered with a fluorescent material such as various phosphors. An insulating layer 5 is formed at region other than the anode segments. Grid electrode 10 including a fine wire mesh is placed thereon in a predetermined interval. Tungsten filaments 12 are suspended above the grid electrode by a suitable supporter, so that they can emit electrons toward the anode segments. Thereafter, a spacer 6 is placed on substrate 4 and an upper substrate 2 is placed on spacer 6 which provides a predetermined space between and around the glass substrates 2, 4, thus forming an evacuated envelope 7 to complete the display tube.
However, the length of filament may vary due to heat generated from the electric current during operation. Such variations mean that the interval between grid electrode 10 and filaments 12 cannot be consistently uniformly kept, thereby causing a poor display created by anode segments 8. Thus, the filaments 12 should be suspended in tension so as to be held above and across electrode 10. A metal supporter is used to thereby solve this heat/expansion problem.
A conventional metal supporter 14 is described in connection with FIG. 4A showing its general form.
A plurality of extensions 142 extend upwardly from a surface adjacent one side edge of a plate 140 of the supporter. The end of each filament 12 is welded to and fixed on top of each extension 142.
FIG. 4B shows another form of a supporter 14a.
A plurality of extensions 144 extend upwardly from plate 140 and have a self-elasticity, to apply some tension to the filaments 12 welded to their tops.
However, the connection by welding both ends of the filaments to the supporters causes the change in quality of the filaments due to heating during welding thereby forming a weak part against the tension. Further, an equipment for welding together with a means for placing the filaments on the exact place for welding are required thereby resulting in the complex structure.